hackport will now attempt to guess which minimum ghc version is required to
compile packages merged. If it fails it will use the default GHC dependency.
It looks at all the dependencies, trying to identify all core packages used.
As we don't allow upgrading of core packages, using this information we can
sometimes make the dependency to ghc stricter.
Also, hackport will not generate dependencies to cabal which are lower than
what was shipped with the GHC version hackport knows the package requires.